BEID (Omicron-1 Eridani). In the middle of the first southerly turning of Eridanus (the River) lie a seeming pair of fourth magnitude stars, Beid and Keid, which Bayer placed far down in the Greek alphabet as Omicron-1 and Omicron-2. The part of Eridanus that contains Azha (Eta Eridani) and stars south, where the River makes its second southerly plunge, were known to the Arabs as the "Ostrich's Nest." Outlying Beid refers to "the Eggs" and Keid to the "Egg Shells," all showing a wonderful celestial mixture of cultures. Though to the eye they may look like a real pair, they are very much not. Keid (Omicron-2) is very close, a mere 16.5 light years away, where as Beid (Omicron-1) lies over 7 times farther, 125 light years distant. To be roughly similar in apparent brightness, Beid (mid-fourth magnitude, 4.04) must be much the more luminous. Indeed, it is a white class F (F2) giant, with a temperature of 7100 Kelvin and a luminosity 28 times that of the Sun. Though direct measure of mass is not possible (it has no known companion, whose orbit would determine the mass), its current luminosity and temperature compared with theory show its mass to be almost exactly double that of the Sun. The star seems to have recently left the core- hydrogen-fusing main sequence of stars, and is now evolving with a dead (for now) helium core that is surrounded by a shell of fusing hydrogen. With a radius of 3.5 times that of the Sun, its rather high equatorial rotation speed of 100 kilometers per second show the star to rotate in less than 2 days. Beid's real distinction, however, is that it is a notable variable, a " Delta Scuti" star, the variation discovered only in 1971. Delta Scuti stars are less- luminous versions of the Cepheid pulsating variables (those like Mekbuda). Unlike simple Cepheids, however, they pulsate very subtly and rapidly with several periods at the same time. Beid varies by at most only a few hundredths of a magnitude (a few percent, invisible to the naked eye) with a principal period of 1.8 hours, another of about 3.5 hours, and certainly yet more. Such variations are extremely difficult to study and are often quite uncertain. As Beid evolves to a cooler giant, the instability will eventually cease.
Written by Jim Kaler. Return to STARS.